In the latest glitch, investors wishing to buy shares in popular investment trusts such as Brunner and Merchants discovered that these investment companies have been erased from Smart Investor's trading systems.

Barclays has now admitted that these investments were absent from the platform for almost six weeks, despite complaints from customers and queries being raised by the companies concerned.

At least one client who telephoned Smart Investor was wrongly told that these investments had been removed due to their failure to comply with new European legislation.

Allianz Global Investors, which manages both Brunner and Merchants, has refuted this, claiming its paperwork was "submitted in good time".

It said: "We have been contacted by a number of shareholders complaining that they are unable to purchase further shares in Brunner and Merchants via Smart Investor. Unfortunately Barclays has created this problem."

Rival brokers, including Hargreaves Lansdown, confirmed their clients have been able to buy shares in these investments as usual.

Telegraph Money contacted Barclays on the morning of February 9 and the trusts were available back on the site later that afternoon.

Barclays said: "Our data and platform providers experienced a problem transmitting data for a small number of investment trusts. This has now been fixed. This was our issue, and was in no way the fault of Allianz or Baring.”

Barclays Stockbrokers' glitchy switch to Smart Investor

The changeover from Barclays Stockbrokers to Smart Investor took place in August 2017 and resulted in immediate problems, with some customers unable to log in and others battling to find their way around the new site.

Some complained that because Smart Investor was bolted on to the bank's main current account system deluged them with irrelevant information about mortgages and other services.

A further problem is that customers who wish to contact Smart Investor staff or complain - but who do not have current accounts with the bank - are also barred from using online chat or email messaging. This has forced customers to use the telephone service, which has involved long waits.

Barclays acknowledged this last failing and promised to offer messaging and chat services "within weeks".